This invention is directed to devices for measuring the weight of a load at the end of a rope wrapped over a rod.
It is known to measure the weight of a load at the end of a rope wrapped or trained over a rod by using four strain gauges configured as a bridge circuit glued inside a central bore 3 in a rod 4. This is an optimum configuration to measure a force applied in a vertical direction only. When the force to be measured has a constant angle relative to the strain gauge, one set of gauges has been used.
The calibration procedure used in this instance is as follows.
1. Place the instrumented rod in a calibration fixture that applies a load in the vertical direction.
2. Apply an increasing load in a series of load steps. The maximum load is the expected maximum load the sensor will see. Measure the strain gauge bridge output at each of these load steps.
3. Repeat step 2 for decreasing load steps to zero load.
4. A curve is then fitted to this data, for example, a simple linear curve y=mx+b. Then for any given x, the output y can be calculated. If the slope m has units of pounds/mV and assuming b=0 lbs., then we can directly relate the output of the bridge circuit to the applied load P. A linear fit is known to give an overall accuracy of 1% full scale.
It is also known to use two strain gauge bridge circuits disposed to intersect each other at right angles to measure tensile or compressive strains in biaxial directions. This is known as a bi-axial load-sensing element.